Two-stage fluid coking

ABSTRACT

A two-stage fluid coking process in which the first stage is a transfer line for short contact time and the second is either a transfer line or a fluidized bed.

United States Patent Saxton [4 1 June 20, 1972 s41 TWO-STAGE FLUIDCOKING 2,894,899 7/1959 Crawley ..2o8/127 2,863,823 12/1958 Moser.....208/92 [723 57'' wanen' 2,983,671 5/1961 Fogle..; ..208/127 [73]Assignee: Esso Research and Engineering Company 3,459,655 8/ 1969Kimberlin et a1. ..208/127 2,862,871 12/1958 Smith ..208/53 Flled= (M20, 1969 2,868,719 1/1959 Martin et aL. ....208/157 2 APPL No: 67 4912,761,825 9/1956 Schultz ..Z08/46 Primary ExaminerDelbert E. Gantz [52]US. Cl ..208/ 127, 208/50, 208/ 160 Assistant E in -G, E, Sch itkons [51 Int. Cl ..C10b 47/24, C10b 55/10, ClOg 9/32 A m P a an and stahl andC, D. Stores [58] Field of Search ..208/127, 50, 46, 160

57 ABSTRACT [56] References Cmd A two-stage fluid coking process inwhich the first stage is 8 UNITED STATES PATENTS transfer line for shortcontact time and the second is either a transfer line or a fluidizedbed, 2,702,267 2/1955 Keith ..208/127 2,736,687 2/ 1956 Burnside et a1...208/127 14 Claims, 2 Drawing figures sacommav coxme ZONE COKE BURNERCOKE PRODUCT PATENTEnJum m2 SHEET 2 OF 2 l 1 llllllllllllllialr CRACKEDPRODUCTS COKE TO STRIPPER OR BURNER HOT COKE FROM mm mm A s V w. R n W NT. m R A RESIDUAL FEED PRIMARY CRACKING ZONE HOT RECYCLE GAS BACKGROUNDOF THE INVENTION This invention relates to an improved petroleumrefining process involving improvements in fluid coking for upgrading ofheavy oils. More specifically the invention relates to a method forpreventing coke buildup in undesired locations, and-at the sametimeproviding greater control of product yield distribution.

As is well known in the art, the fluid coking process uses a fluidcoking vessel or reactor" and an external heating vessel, e.g. a fluidbed burner. A fluid bed of solids, preferably coke particles produced bythe process having a size in the range of about 40 to L000 microns, ismaintained in the coking zone by the upward passage of a fluidizing gas,usually steam. The temperature of the bed is maintained at about950-l,050 F. by circulating solids (coke) to the heating vessel (cokeburner) and back. The heavy mineral oil to be converted is injected intothe fluid bed and upon contact with the hot solids undergoes pyrolysisevolving lighter hydrocarbon vapors and depositing coke on the solids.The turbulence of the fluid bed normally results in substantiallyisothermal conditions and in thorough and rapid distribution of theheavy injected oil. Product vapors, after heavy entrained solids areremoved, are withdrawn overhead from the coking vessel and sent to ascrubber and fractionator for cooling and separation. Generally. astream of the coke particles is continuously withdrawn from the cokingvessel or reactor and passed to the burner, where some of it is burnedto heat the remainder, and heated coke is continuously recirculated tothe reactor.

In such a process many of the operating and maintenance problems arecaused by coke formation in unwanted locations, such as feed nozzles,reactor vessel walls, and in the cyclone system. Under normal operatingconditions the coke buildup rates are very slow, allowing continuousoperations for 18 months or more. However when abnormal operations occuror when coke circulation is temporarily slowed or stopped severebuildups may occur.

Coking of equipment surfaces and particle agglomerization are causedprimarily by the high molecular weight fragments produced by very rapidprimary cracking of the full residuum feed and by the polymerizationproducts produced by slow secondary reactions including polymerizationof some of the primary products and further cracking of other primaryproducts. Coke formation is aggrevated in areas where temperature orturbulence are reduced for any reason below critical values.

Present coker reactors must be operated within a relatively narrow rangeof conditions which limits the degree of control over product yielddistribution and over product qualities. This limitation is ofrelatively minor importance in most current plants where the primaryobjective is to convert heavy residuum. and the more valuable gaseousand liquid products are a relatively small percentage of the totalvolume of similar products in the complete refinery. However, improvedproduct flexibility would be a considerable asset to the process, and isparticularly important in refineries processing very heavy crudes suchthat the coker products have a major influence on overall refineryyields.

SUMMARY OF THE INVENTION The main objects of this invention are toimprove the dispersion of hydrocarbon feed in the circulating coke andto separate the reaction system into two zones in which crackingconditions can be adjusted to minimize unwanted coke deposits on theequipment, and to achieve desirable changes in product yields andqualities.

These and other objects are accomplished by providing a two-zone reactorfluid coking system in which the primary cracking reactions take placein a transfer line and the secondary reactions occur either in a secondtransfer line or in a dense bed into which the primary zone discharges.A high degree of uniform turbulence and independent control overtemperature can be accomplished in both cracking zones.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticrepresentation of one embodiment of the invention involving a two-stagecoking process in which the primary zone is a transfer line reactor andthe secondary zone is a dense bed of fluidized coke particles.

FIG. 2 is a diagrammatic representation of another embodiment in whichboth the primary and secondary coking zones are transfer line reactors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG.1, a heavy residual oil having a Conradson carbon number generallybetween 20 and 50 (but not limited to this range) is introduced into thebottom of primary coking zone 1 through line 2 and atomized with about vmol of steam or recycled light gases from the process or the like perbarrel of feed in line 3 and nozzle 4. Primary coking zone 1 is atransfer line reactor, suitably having a length of 20-40 feet whichtogether with a flow of about 10 lbs. of coke per cubic foot of gas atthe bottom to 2.5 lbs. of coke per cubic foot of gas at the top affordsa residence of time of l to 2 seconds in the primary cracking zone. Hotcontact material from a coker burner (described below) is introducedthrough line 5 and maintains the temperature in the primary reactionzone at about 900-l,000 F. It should be noted that the above describedconditions are illustrative of a representative operation, but are notintended to be restrictive. For example, if a high yield of lightolefins is desired, the temperature may be in the range of l,500 tol,800 F. with less than I second residence time. Alternatively, muchhigher residence times, up to about 10 seconds may be desired in somecasesfor example, for cracking of very refractory materials, such asrecycled coker gas oil or other hydrocarbon which has already passedthrough a prior thermal or catalytic cracking step.

In this example the contact material is coke. However, other materialscan be used which may or may not be catalytic in nature. Examplesinclude such commonly used materials as silica, alumina, zirconia,magnesia, combinations of these such as silica-alumina, silica-magnesia,etc. The materials may be synthetically prepared or may be naturallyoccurring materials such as pumice, clays, kieselguhr, diatomaceousearth, bauxite, and the like. When it is desired to use a contactmaterial having catalytic properties the material will generallycomprise silica-magnesia or other catalytically active combinations ofinorganic oxides and when it is desired that hydrogenation activity beimparted to the catalyst other material may be added such as smallquantities of metals having hydrogenating activity including nickel,cobalt, iron, molybdenum, vanadium, chromium, platinum, palladium, etc.,compounds of these such as their oxides or sulfides or any combinationsthereof. Since no net coke make need be realized, a heterogeneouscontact material comprising a refracto: ry oxide base stable at the hightemperature herein contemplated may be used.

The contact of the residua and hot contact material results in the pitchbeing converted to coke which deposits as thin layers on the hotparticles, and lower boiling hydrocarbons which pass overhead in thevapor state. The reactions occurring can be grouped in two generalclasses. First there is the very rapid primary cracking of the fullresiduum range to form a broad spectrum of products from H and CH tovery high molecular weight fragments. These will include large amountsof unsaturated and unstable compounds as well as some very refractorymaterials. Second there are the slow secondary reactions includingpolymerization of some of the primary products and further cracking ofother primary products. These secondary reactions are held in checksomewhat by the relatively low temperature and short residence time inthe primary reactor thus minimizing coke formation since coking of theequipment surfaces and particle agglomeration are caused primarily bythe high molecular weight fragments from the primary cracking andsubsequent polymerization of the products from the secondary reactions.Coke deposition on equipment surfaces is further minimized by thescouring action of the particles which are carried up in a suspendedstate at velocities ranging from about 10 to 40 ft/sec.

In any case the contact of the residuum and the hot contact materialresults in the residuum being converted to coke and lower-boilingvaporous hydrocarbons.

The primary coking zone 1 discharges into the bottom of secondary cokingzone 6 containing a dense fluidized mass of particles of contactmaterial having a level L, and resting on distributing grid 7 throughwhich hot fluidizing gas is distributed and which is introduced into thebottom of the secondary zone below the grid 7 through line 8. Thisfluidizing gas may suitably be hot recycle gas from the processes,steam, or any other suitable fluidizing gas. This gas serves severalfunctions: it assures good fluidization thereby minimizing temperaturegradients which could lead to coke deposits in cool zones; it provides ameans of controlling the gas atmosphere and thereby influencing productyields from cracking; it also serves to add heat to this zone. Thetemperature of this secondary zone is preferably slightly higher thanthat in the primary zone, namely, about 920] ,050 F. and the residencetime is typically 1 to 10 seconds.

A stream of fluidized coke is transferred from secondary zone 6 throughline 9 to a coke burner 10 where a portion of the deposited coke layeris burned to generate heat for the process. A conventionalcountercurrent stripper (not shown) may be provided to remove anystrippable hydrocarbon from the coke leaving the secondary coking zone.Combustion air is supplied through line 11. Heat is returned to thereactor in a recycled coke stream (lines 5 and 13) which is normallyabout to 200 hotter than the coke stream from the reactor to the burner.The recycled coke temperature, which equals the burner bed temperature,is normally controlled by regulating the quantity of combustion air. Therelative quantities of recycled coke through line to the primarycracking zone and through line [3 to the secondary cracking zone areregulated as required to maintain the desired cracking temperatures inthe two zones. Another consideration affecting the distribu tion ofrecycled coke to the 2 cracking zones is the ratio of particle surfacearea to residuum feed in the primary zone. The particle surface must besufi'icient that the atomized liquid residuum feed can be distributed ina thin film over the surface, thus avoiding excessive agglomeration ofparticles in large liquid droplets. In a normal case this is not alimiting consideration relative to the mass flow rate of particlesrequired to supply the reaction heat. Typically, the mass flow ratio ofhot coke to residuum feed will be in the range of 3/1 to lO/l.

The net coke product, which equals the gross coke deposited on theparticles in the reactor minus the coke burned in the burner, iswithdrawn from the burner through line 15.

Vaporous products and entrained solid particles pass from the upperportion of secondary coking zone 7 into cyclone separator 16 where solidparticles are separated and drop into the fluidized bed through line 17.Vapors leave through line 18.

Referring now to FIG. 2, primary cracking zone 101 is identical with theprimary zone of FIG. 1 and operates in the same manner. instead ofdischarging into a dense fluidized bed in a secondary coking zone as inFIG. 1, it discharges into a transfer zone 106 which is slightly longerthan transfer line 101 so as to increase the residence time. Typicallytransfer zone 106 will provide from 1 to 5 seconds residence time. Hotrecycle gas is introduced into the bottom of this zone through line 108to add heat and to control the reaction time and the atmosphere in whichcracking occurs. The mixture of coke laden contact particles and gasdischarge from secondary zone 106 against deflecting hood 107 and fallinto dense bed 109 which is maintained in a fluidized state by hotrecycle gas or other fluidization and stripping gas introduced throughlines 110, Ill and 112. Hot coke-laden particles are withdrawn from thebottom of fluidized bed 109 through line 113 and passed to a burner (notshown) which is identical with the burner 10 of FIG. 1. Cracked productsare withdrawn overhead through line 118.

From the above description it is evident that an improved coking andcracking process has been developed which minimizes deposition of cokeon equipment surfaces, prevents bogging or excessive agglomeration ofsticky particles, and gives improved yields of products. The system willhave more uniform and predictable fluidization characteristics thanconventional single stage coker reactors which will result in improvedoperability. The particle size of the circulating coke can be allowed toincrease which will reduce or eliminate the need for forced attrition.Present plants utilize steam jet attriters as a supplement to normalattrition within the systemto generate seed coke particles which replacethe particles withdrawn as product or blown overhead, and to reduceparticle size for operability reasons.

The conditions described in the above example are t pical of anoperation in which the primary objective is to obtain a maximum yield ofliquid products from a residuum feed. Other hydrocarbon feeds can beprocessed and conditions can be varied widely, within the scope of thisinvention, to accomplish widely different yield patterns. For example,the reactor can be operated at 1,500 to 1,800, with a correspondinglyhigher burner temperature, to maximize the yield of H and lighthydrocarbon gases. The yield of unsaturated compounds can be maximizedby a combination of elevated temperatures and steam dilution asinjection and fluidizing steam. Use of H rich gas for injection andfluidization will minimize the dehydrogenation of gaseous and liquidproducts, thereby, yielding maximum saturated" products.

The process described can be operated over a wide range of pressure. Themost normal pressure is from about 15 to 50 psig. However, it is wellknown that operating pressure is an important parameter affectingcracking process results. Operating pressure also affects physicalequipment sizing and overall economics. Thus, in particular situations,it may be desirable to design a two-stage fluid coking system asdescribed herein, for operations at elevated pressures in the range of200-300 psig.

The present invention having thus been fully described and illustrated,what is claimed as new, useful and unobvious and desired to be securedby Letters Patent is:

l. A process for cracking heavy hydrocarbon material to coke and lightergaseous and liquid hydrocarbon products, which comprises:

contacting said heavy hydrocarbon material with hot particulate solidsin a first reaction zone operating at cracking pressures andtemperatures to produce said lighter products, said contacting being fora period of the time to enable cracking of said heavy hydrocarbonmaterial to occur under said cracking pressures and temperatures, butnot for a period of time to enable substantial amounts of coking andparticle agglomerization within said first reaction zone;

passing said hydrocarbon material that is not cracked in said firstreaction zone, said hot particulate solids, said lighter products andcoke into a second reaction zone operating under cracking conditionsmore severe than in said first reaction zone to enable more of saidhydrocarbon material to be cracked to more lighter products; and

recovering said lighter products from said second reaction zone.

2. A process according to claim 1 wherein said cracking conditions insaid second reaction zone is more severe than in said first reactionzone in that said cracking temperature in said second reaction zone ishigher than in said first reaction zone.

3. A process according to claim 2 wherein said cracking temperature insaid second reaction zone is between about 20 and about 50 F. higherthan in said first reaction zone.

4. A process according to claim 1 wherein a portion of said heavyhydrocarbon material is atomized before contacting with said hotparticulate solids.

5. A process according to claim 4 wherein said heavy hydrocarbonmaterial is atomized by contacting with a stream of steam or recycledlight gaseous products which have been recovered from said secondreaction zone.

6. A process according to claim 5 wherein said atomizing and contactingoccur simultaneously in said first reacu'on zone.

7. A process according to claim 6 wherein said oil is atomized bycontacting with a stream of steam or recycled.

said lighter products, said contacting being for a period of timebetween about 0.5 and about 10 seconds;

passing said remaining oil, said hot particulate solids, said lighterproducts and coke into a second reaction zone operating under crackingconditions more severe than in said first reaction zone to enable moreof said oil to be cracked to more lighter products; and

recovering said lighter products from said second reaction zone.

9. A process according to claim 8 wherein a portion of said oil isatomized before contacting with said hot particulate solids in saidfirst reaction zone.

10. A process according to claim 8 wherein a portion of said oil isatomized simultaneously with said contacting in said first reactionzone.

1 l. A process according to claim 8 wherein said contacting in saidfirst reaction zone which is a transfer line reactor being for a periodof time between about l and about 2 seconds.

12. A process according to claim 11 wherein said second reaction zone isa transfer line reactor, and said oil is contacted with said hotparticulate particles for a period of time between about I and about 5seconds.

13. A process according to claim 8 wherein said cracking conditions insaid second reaction zone is more severe than in said first reactionzone in that said cracking temperature in said second reaction zone ishigher than in said first reaction zone.

14. A process according to claim 13 wherein said cracking temperature insaid second reaction zone is between about 20 and about 50 F. higherthan in said first reaction zone.

2. A process according to claim 1 wherein said cracking conditions insaid second reaction zone is more severe than in said first reactionzone in that said cracking temperature in said second reaction zone ishigher than in said first reaction zone.
 3. A process according to claim2 wherein said cracking temperature in said second reaction zone isbetween about 20* and about 50* F. higher than in said first reactionzone.
 4. A process according to claim 1 wherein a portion of said heavyhydrocarbon material is atomized before contacting with said hotparticulate solids.
 5. A process according to claim 4 wherein said heavyhydrocarbon material is atomized by contacting with a stream of steam orrecycled light gaseous products which have been recovered from saidsecond reaction zone.
 6. A process according to claim 5 wherein saidatomizing and contacting occur simultaneously in said first reactionzone.
 7. A process according to claim 6 wherein said oil is atomized bycontacting with a stream of steam or recycled light gaseous productswhich have been recovered from said second reaction zone.
 8. A processfor cracking a heavy residual oil having a Conradson carbon numbergenerally between 20 and 50 to form coke and lighter gaseous and liquidhydrocarbon products, which comprises: contacting said oil with hotparticulate solids in a first reaction zone operating at a crackingtemperature between about 900* and about 1,000* F. and at a crackingtemperature between about 15 and about 50 psig to produce said lighterproducts, said contacting being for a period of time between about 0.5and about 10 seconds; passing said remaining oil, said hot particulatesolids, said lighter products and coke into a second reaction zoneoperating under cracking conditions more severe than in said firstreaction zone to enable more of said oil to be cracked to more lighterproducts; and recovering said lighter products from said second reactionzone.
 9. A process according to claim 8 wherein a portion of said oil isatomized before contacting with said hot particulate solids in saidfirst reaction zone.
 10. A process according to claim 8 wherein aportion of said oil is atomized simultaneously with said contacting insaid first reaction zone.
 11. A process according to claim 8 whereinsaid contacting in said first reaction zone which is a transfer linereactor being for a period of time between about 1 and about 2 seconds.12. A process according to claim 11 wherein said second reaction zone isa transfer line reactor, and said oil is contacted with said hotparticulate particles for a period of time between about 1 and about 5seconds.
 13. A process according to claim 8 wherein said crackingconditions in said second reaction zone is more severe than in saidfirst reaction zone in that said cracking temperature in said secondreaction zone is higher than in said first reaction zone.
 14. A processaccording to claim 13 wherein said cracking temperature in said secondreaction zone is between about 20* and about 50* F. higher than in saidfirst reaction zone.